At the centre of our galaxy, a black hole could be about to chow down on one of the biggest meals it's ever had - a mysterious gas cloud around three times the mass of Earth.

A montage of the galactic centre by the Swift X-ray Telescope from 2006-2013. Credit: Nathalie Degenaar

Astronomers using NASA's Swift telescope are waiting with bated breath to see if the cloud, known as G2, will be sucked into the supermassive black hole in a collision that will not only be a spectacular fireworks show, but could also help boffins see how fainter black holes feed in comparison to their brighter counterparts.

"Everyone wants to see the event happening because it's so rare," said Nathalie Degenaar, who leads the imaging effort as a Hubble research fellow in the Department of Astronomy at the University of Michigan.

G2 was discovered by astronomers in Germany in 2011 and was expected to hit the black hole Sagittarius A* (Sgr A) late last year. The smash hasn't happened yet, so boffins are now predicting an impact in the next few months.

Sgr A is 26,000 light years away from us near the constellations Sagittarius and Scorpius and is the Milky Way's central supermassive black hole. Compared to other collapsed stars at the centres of elliptical and spiral galaxies, Sgr A is dim, despite having a mass at least four million times that of the Sun.

"Given its size, this supermassive black hole is about a billion times fainter than it could be," Degenaar said. "Though it's sedate now, it was quite active in the past and still regularly produces brief X-ray flares today."

"We think that the fainter ones are the majority, but it's very difficult to study those. We just can't see them. Ours is the only one we can study to understand what their role is in the universe."

The gas cloud collision should give astronomers the rare chance to see how fainter supermassive black holes feed and if they consume matter in the same way as bright ones. Obviously, black holes themselves are invisible, but the material falling into them shines in X-rays.

The Swift observatory is the only telescope providing daily updates at X-ray wavelengths of where the crash will most likely be and will be posting images online here.

Astroboffins will be looking for a change in brightness to indicate the black hole is starting its meal, but they don't know yet just how luminous it will be as they have yet to figure out what kind of gas cloud G2 is. If it's all gas, the region could glow for years to come as Sgr A slowly swallows it. But if G2 is hiding an old star, the display could be less dramatic, with the black hole only getting a few sips of the gas while the star passes by, dense enough to escape the event horizon.

"I would be delighted if Sagittarius A* suddenly became 10,000 times brighter. However it is possible that it will not react much—like a horse that won't drink when led to water," said Jon Miller, an associate professor of astronomy who also works on the project.

"If Sagittarius A* consumes some of G2, we can learn about black holes accreting at low levels—sneaking midnight snacks. It is potentially a unique window into how most black holes in the present-day universe accrete." ®